Dry blasting agents, such as ammonium nitrate/fuel oil (ANFO) explosive compositions, are commonly used for blasting in open pits, underground mining, quarry and construction. As ammonium nitrate is not readily detonatable in and of itself, it is typically mixed with a carbonaceous fuel oil in order to obtain a mixture that is more or less oxygen balanced and therefore detonatable.
An ANFO explosive composition could be prepared at a mine and immediately loaded into a series of boreholes. Alternatively, it could be manufactured at a production site where it is packaged in a polyethylene or a “kraft” paper bag (e.g., having a wax lining) and stored for future use. The ANFO explosive composition would be loaded into the boreholes (typically from about 10 to 15 holes to more than about 100 holes) over a period of days. Typically, an ANFO explosive composition may be kept in a borehole anywhere from 1 hour up to 14 days prior to being detonated. If the explosive is a prepackaged ANFO explosive composition, then due to shipping and handling time, the explosive composition must be stable for extended periods of time. In some cases, the length of time between mixing the explosive composition and detonation of the explosive composition may be up to 180 days or more.
A problem that has been encountered with prior art high density ANFO explosive compositions is that the fuel oil tends to separate from the ammonium nitrate during the time that the explosive composition is stored in the borehole awaiting detonation. If the oil separates from the ammonium nitrate, the explosive composition may deflagrate or it may fail to explode.
The density of ammonium nitrate particles that are used in an ANFO explosive composition may vary widely. Higher density ammonium nitrate particles have a greater bulk density and, when mixed with fuel oil, produce a higher detonation pressure. However, as the density of the ammonium nitrate particles increases, the oil adsorption of the particles decreases. This is due to the lower porosity of higher density ammonium nitrate particles. The lower porosity results in oil migrating to the bottom of a borehole. Therefore, it is difficult to make a storage stable explosive composition with higher density ammonium nitrate prills (e.g., about 0.86 to about 1.1 g/cc, or higher).
One method to reduce oil segregation with higher density ammonium nitrate particles is to use high viscosity fuel oils. However such fuel oils are expensive, not always available in remote locations where ANFO explosives are used and require costly shipping.